1. Field of the Invention
This invention relates to a standard for spectral reflectance for use in calibrating an automated infrared reflectance analyzer for quantitation of constituents of interest of samples.
2. Prior Art
Prior known standards for spectral reflectance for use in infrared reflectance measurements of the general type performed by the analyzer described in Anson et al. U.S. Pat. No. 3,776,642 have been structured of Teflon polytetraflouroethylene resin of either powdered or solid form, powdered barium sulfate, powdered magnesium oxide and a nickel alloy having a porous reflective surface. Such powders, which are used in unbound and uncoated form, exposed to the ambient atmosphere in use, have adsorbed thereon from the latter substances such as dust, oils, tars and oxides of nitrogen and sulfur and other chemical substances, for example, which adversely affect the usefulness of such powdered substances as standards by a change in the ratio of the incident light to the reflectivity of such light from such substances. Obviously, such standards in powdered form cannot be cleaned readily of such foreign substances. A coated surface of magnesium oxide has been used as such a reflective standard. For this purpose, magnesium ribbon has been burned under a support surface. The resulting vapor containing magnesium oxide then coats the support surface. Such a standard consists of magnesium oxide which is fragile and difficult to clean.
Inasmuch as such analyzers have extensive use at rail and truck terminals and grain storage facilities as well as other places, as brought out in the aforementioned Anson et al U.S. Patent, where the environment is dusty and polluted with chemicals these standards in the form of powder usually require replacement after a relatively short period of use, say one day to a week, depending on environmental conditions.
Further, such standards of exposed, powder form, like all other standards for spectral reflectance, must present a reflective surface which is smooth and uniformly flat in a plane normal to the incident radiation. The unbound, exposed powder may shift, though compacted to at least some extent, even in normal handling, particularly as movement of the standard from registry with the incident light to a position out of such registry is required in such analysis to permit the position of the sample to be switched with respect to the registry position of the standard. Such shifting of the powder in the standard changes the reflective characteristics of the standard and the reflective surface thereof must be resmoothed as with a spatula.
Exposed barium sulfate and magnesium oxide in the required dehydrated form absorb moisture which adversely affects the reflectance characteristics of such powders as standards by changes in the ratio of incident light to reflected light. These powders require frequent drying out in an oven which is costly and inconvenient. Humidity is also known to adversely affect at least the use of certain ones of these powders as such standards by a change in the aforementioned ratio, and the reflective surface area presented by all of the aforementioned powders lacks the desired degree of homogenity.
An attempt to hermetically seal such standards to protect them from moisture and foreign substances, such as by enclosure in glass which by interfacing with the powder tends at least to some extent to maintain a degree of material flatness and smoothness of the powder, has been found to interfere with the desirable diffuse reflectance of the powder and results in a more specular and less diffuse reflectance caused by the glass cover. Further, such glass-powder interface may develop interference fringes altering the reflective characteristic of the standard.
In the use in the prior art of solid or powdered Teflon resin as an infrared standard of reflectance in accordance with the aforementioned Anson et al. Patent, the material was found to be relatively transmissive of light and when used as a reflective standard, incident light on a surface thereof has shown a marked tendency to be reflected or absorbed by a support for such standard in opposing relation to the surface of the standard opposite the first mentioned surface thereof. This is a disadvantage in its use as such a standard, particularly noticeable in the usual approximately 3/8 in. thickness of solid Teflon material supplied for such use and even more noticeable in lesser thicknesses. In addition, for such uses as a standard in such analyses, the solid Teflon material must be machined to provide the requisite degree of material flatness and smoothness of the reflective surface in a plane normal to the incident light. Such surface must be perfectly smooth and without scratches or other nonuniformities. The softness of such Teflon material renders it easily marred and therefore damaged as to its reflective characteristics as by wiping with a cloth and even more by scrubbing to remove deposits of dust and chemicals deposited thereon in use as previously described. For this reason attempts have been made to clean the reflective surface of such material by spraying substances thereon such a Freon fluorochlorohydrocarbon but such cleansing results in residues of cleaning substances being left on such reflective surface which adversely affect the ratio of incident light to light reflected from the surface by a change in such ratio. Neotec Corporation of Rockville, Md., supplies a standard for reflectance known as a "white tile." It is a relatively thin square comprising solid Teflon polytetraflouroehylene and not, as its name suggests, a ceramic material.
A standard for spectral reflectance for use in the analyzer of the Anson et al. U.S. Pat. No. 3,776,641 should exhibit light reflectance equally throughout the operational wavelength range of the analyzer. Additionally, the light absorption of the standard should be unvarying throughout this wavelength range. These properties are hereinafter referred to as "optical flatness." If the percentage of incident light absorbed by the standard at a particular wavelength in the operational range is higher, there is a resultant decrease in the percentage of light reflected and a deviation from optical flatness. All the aforementioned prior art standards for spectral reflectance in varying degrees do not exhibit this optical flatness, without consideration of such factors as service life described above. Of various prior art standards tested at the instigation of the assignee of the present invention, a porous nickel alloy surface came closest to such optical flatness in the last-mentioned operational range and, more precisely and restrictively, in the range of 1.4 -2.4 micrometers. However such nickel alloy standard is subject to the drawbacks that, at best, it is extremely difficult to duplicate in fabrication in a manner to obtain the same reflectance characteristics, similar to other prior art reflectance standards discussed hereinbefore. It clogs with dust and cannot be cleaned easily and quickly.
Teflon polytetrafluoroethylene resin, in either powder or solid form, exhibits reflection peaks in the region of 2.15 micrometers which is the wavelength region most important in the determination of protein in samples. Substantial change in the characteristics of spectral reflectance of the material of such standard in this region was noted as being attributable to such reflection peaks. Other aforementioned materials of the aforementioned prior art standards have varying light reflection characteristics over such operational wavelength range from lot to lot because of the difficulty of uniform manufacture.
The present invention overcomes these difficulties with such prior art reflectance standards.